Maximum thrust and operating efficiency of a gas turbine engine is achieved when the engine exhaust passes through an exhaust nozzle which controls the expansion of the exhaust gases. Maximum operating efficiency generally requires that the nozzle be configured to exit the exhaust stream at substantially the same pressure as the surrounding ambient atmosphere.
With aircraft operating both at subsonic and supersonic speed the exhaust nozzle pressure ratio varies over a wide range. Variable throat convergent/divergent nozzles are used to achieve proper operation for the various operating conditions.
These gas turbine engine nozzles operate with gas at an extremely high temperature level such that cooling of the nozzle surfaces is required. A known method includes passing a forced flow of cooling air through a liner which is lining both the convergent flaps and the convergent seals. This cooling flow is then dumped at the throat adjacent to the surface of the divergent section to film cool that section.
Various amounts of cooling air are required to properly cool the divergent section at various operating conditions. In accordance with conventional practice however the air flow is established based on the maximum airflow which is required under the most adverse condition. This forced air flow continues to operate at other conditions where the full amount is not required for cooling. This excess use of cooling air takes air away from the combustor and gas turbine thereby decreasing the efficiency of the engine.